Rechargeable electrochemical cells are designed for charging to store input electrical current as energy, and discharging for outputting or releasing the stored energy as output electrical current. Each battery chemistry has particular advantages and disadvantages with regards to energy density, power density, efficiency, rechargeability, cost and so on. Effective combination of different battery chemistries to produce a hybrid battery may offer significant advantages which may be unavailable with a single battery chemistry. Typically, hybrid batteries may be provided as sub-cells each having a separate housing and ionically conductive medium relevant to a particular battery chemistry. An example of a hybrid system using a common electrolyte is shown in U.S. Patent Publication No. 2014/0272477 A1 (now U.S. Pat. No. 9,048,028) which discloses an alkaline electrolyte shared between a nickel-metal hydride cell and electrochemical capacitor to create a hybrid capacitor-battery system.
Metal-air batteries offer significant advantages in terms of energy density, as unlike most battery chemistries in which store oxidant is stored at the cathode, metal-air batteries use oxygen from the air as a source of oxidant. The existence of a continuous and virtually limitless oxidant source enables, in principle, high energy densities.
U.S. Patent Publication No. 2011/0250512 A1 (now U.S. Pat. No. 9,761,920) also shows a system designed for increased efficiency during shorter charge/discharge cycles, and higher storage ability for longer changer/discharge cycles. That application uses a fuel electrode, which may be coupled to an air electrode or a nickel electrode to create different cell behaviors in a common electrolyte.